I hand-pick the hornworms and drop them into a cup of soapy water. I did this last year when I first discovered them. They have not returned this year. I also got some unexpected help in hornworm control from the parasitic wasps. If the hornworm had the wasp eggs on its body, I left it to let the wasp larvae kill the hornworm. It was also beneficial in growing the parasitic wasp population for the next year.